Suppose that X is a normal random variable and E(X) = −3. Find Var(X) if P(−7 < X < 1) = 0.7888.
For normal distribution, P(X < A) = P(Z < (A - )/
)
E(X), = -3
Var(X), 2
= ?
P(-7 < X < 1) = 0.7888
-3 is the midpoint of the interval (-7, 1) and it is the mean of the distribution.
Therefore P(-7 < X < -3) = P(-3 < X < 1) = 0.7888/2 = 0.3944
P(-3 < X < 1) = 0.3944
P(X < 1) - P(X < -3) = 0.3944
P(Z < (1 - -3)/) +
0.5 = 0.3994
P(Z < 4/) =
0.8994
Take the z score corresponding to 0.8994 from standard normal distribution table
4/ =
1.28
=
3.125
2
= 3.132 = 9.8
Var(X) = 9.8
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